Disease resistance genes are defined as Mendelian factors that cosegregate with the resistance trait. The genes Hm1 and Hm2 control resistance to Cochliobolus carbonum Nelson race 1 and are among the first disease resistance genes to be described. The disease caused by C. carbonum race 1 can be devastating, resulting in yield losses of 80% or more due to plant death and grain mold. In 1938, this disease was first reported in Indiana on an inbred line, Pr (Ullstrup, A. J. (1941) Phytopathology 31, 508-521, which was bred in Iowa from an open-pollinated cultivar, Proudfit Reid (Gerdes, J. T., Behr, C. F., Coors, J. O. & Tracy, W. F. (1993) Compilation of North American Maize Breeding Germplasm (Am. Soc. Agron., Madison, Wis.)). Disease symptoms included grayish tan necrotic spots with concentric rings on the foliage, and infection often resulted in severe molding of the ears and premature killing of the plant. Hm1 and Hm2 are the only disease resistance genes that are known to be fixed at a high frequency in maize germplasm.
Genetic studies revealed that pathogenicity of C. carbonum race 1 is determined by a single locus, Tox2, which also confers the ability to produce HC toxin (Scheffer, R. P. & Ullstrup, A. J. (1965) Phytopathology 55, 1037-1038; Scheffer, R. P., et al., (1967) Phytopathology 57, 1288-1289). HC toxin is a cyclic tetrapeptide of the structure cyclo(D-prolyl-L-alanyl-D-alanyl-L-Aeo), where the unusual amino acid Aeo stands for 2-amino-9,10-epoxy-8-oxodecanoic acid (Gross, M. L, et al., (1982) Tetrahedron Lett. 23, 5381-5384, Walton, J. D., et al., (1982) Biochem. Biophys. Res. Commun. 107, 785-794), and appears to be the sole determinant of pathogenicity. Genetic variants that do not produce HC toxin are unable to colonize much beyond the site of penetration and, therefore, cause only chlorotic or necrotic flecks on leaves (Conistock, J. C. & Scheffer, R. P. (1973) Phytopathology 63, 24-29, Panaccione, D. C., et al., (1992) Proc. NatL Acad. Sci. USA 39, 6590-6594). The Tox2 locus has been cloned and found to encode the enzymes required for the biosynthesis of HC toxin (Panaccione, et al. supra; Scott-Craig, J. S., et al., (1992) J. Biol Chem. 267, 26044-26049). It is not clear how HC toxin allows colonization of susceptible maize. However, by virtue of its inhibitory action on histone deacetylases, HC toxin may interfere with the induction of defense genes in maize, thereby leaving the plant vulnerable to colonization by the pathogen (Brosch, et al., Plant Cell 7:1941-1950 (1995)).
C. carbonum race 1 is one of the most aggressive pathogens of maize. Fortunately, most maize germplasm is resistant. The dominant gene, Hm1 confers complete protection (Ullstrup, A. J. (1941) J. Agric. Res. 63, 331-334; Meeley, R. B., et al., Advances in Molecular Genetics of Plant-Microbe Interactions, 463-467, Nester and Verma (eds.) Kluwer Academic Publishers, Netherlands), and it maps to the long arm of chromosome 1 (1L). Another gene, Hm2, provides partial, adult plant resistance, and it maps to 9L (Nelson, O. E. & Ulistrup, A. J. (1964) J. Hered. 55, 195-199). The cloning of the Hm1 gene has revealed that, by encoding HC toxin reductase (HCTR), Hm1 inactivates HC toxin, and this result is sufficient to prevent infection (Johal, G. S. & Briggs, S. P. (1992) Science 258, 985-987; Meeley, R. B., et al., (1992) Plant Cell 4, 71-77; Briggs, et al., U.S. Pat. No. 5,589,611. The foregoing references are herein incorporated by reference.).
In the present invention the Hm2 polynucleotide sequence has been cloned and sequenced. The Hm2 polynucleotide functions to inactivate HC toxin and other cyclic tetrapeptides such as cyl-2, chlamydocin, apicidin and WF-3161 (Walton, J. D., Biochemistry of Peptide Antibiotics, 179-203, Kleinkauf and Dohren (eds.), Walter de Gruyter Berlin, New York (1990); Darkan-Rattray et al, Proc. Natl. Acad. Sci. USA 93:13143-13147 (1996). The foregoing references are herein incorporated by reference.).
Therefore, the Hm2 polynucleotide can be used to prevent fungal infection in plants or as a selectable marker gene in plant transformation.